Changes in the Frequency of High- and Low-Flow Years in the Permafrost-Dominated Lena River Basin During Long-Lasting Phases of Contrasting Water Flow

Long-term phases (lasting 10–15 years or more) of decreased and increased annual and seasonal runoff and related differences in the frequencies of low-water and high-water river flow years were identified for the Lena across Kyusyur and Tabaga and its tributary, the Aldan in Verkhoyanski Perevoz, for observation periods beginning in the 1930–1940s. Phase boundaries were identified by cumulative deviation curves and the statistical homogeneity of runoff time series. The frequencies of abnormal years were estimated based on the empirical exceedance probability curves, derived from multi-year time series of annual and seasonal flows. Two main long-lasting, quasi-synchronous contrasting phases were revealed on each of the rivers under consideration during an 80-year series of observations for both annual and seasonal runoff. At first, there was a phase of decreased runoff, which was followed by a phase of increased runoff during the period of modern global warming. These contrasting phases exhibit quasi-synchronicity not only within the same river but also across different rivers. However, the duration and magnitude of the flow difference between phases vary both on each river and between them. The phases of increased runoff are characterized by a noticeably higher frequency of high-water years (when runoff is at least 25% of the exceedance probability) and a lower frequency of low-water years (when runoff is at least 75% of the exceedance probability). This inverse relationship has been revealed for the phases of decreased runoff.